lib/repo: Verify txn stagedir existence after locking
lib/repo: Verify txn stagedir existence after locking

This squashes the last race condition I was actively hitting while running
`test-concurrency.py` in a loop. The race is when process A finds a tmpdir to
reuse, and goes to lock it. Meanwhile process B deletes it and unlocks the lock.
Process A then succeeds at grabbing a lock, but the tmpdir is deleted.

Closes: #1352
Approved by: dbnicholson

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree